In SPEC tomography, the patient's body is made to radiate photons, which of course radiate in all directions. To obtain useful information, it is necessary to select photons emitted in well-defined directions by a collimator and to measure the radiation intensity along each path with an appropriate detector. Then, by detecting photons from different directions along closely spaced paths and by using CT scan image reconstruction technology with appropriate modifications, the user can build up a "slice" image of the distribution of photon sources in the body.
The use of SPEC tomography to study brain function has received a strong impetus in the last few years primarily from the introduction of several new radio pharmaceutical tracers that can cross the blood brain barrier and localize in the brain parenchyma.
What is needed to take advantage of the availability of these new tracers is a cost effective high-performance SPEC tomography system. In order to have the resolution of such a system approach that of a positron emmission tomograph system (5-8 mm FWHM), a SPEC tomography system has to have a design optimized exclusively for brain SPEC tomography.
Most of the current SPEC tomographic approaches involve the use of a rotating detector system (scintillation camera or linear detector arrays) to collect information from multiple directions.
These approaches suffer from the problems that arise from complexity in electronic designs, the relative instability of PM tube gains with rotation, and very stringent uniformity requirements for detector response. These problems are analogous to those encountered in the design of the rotate-rotate type CT (third generation). It is well known that the rotate-stationary detectors was designed to avoid these problems. We believe that, for SPEC tomographic imaging, the ring type stationary detector arrangement offers the same advantages that have contributed to the improvements realized by fourth generation CT systems--namely, simplicity, stability, and tolerance to variation in detector responses. What is significant is the fact that most of the disadvantages inherent in fourth generation CT design do not appear to apply to SPEC tomographic systems utilizing stationary detectors. For example, the major criticism of the fourth generation CT design is that only a fraction of the detectors are utilized at any given time. In the stationary detector SPEC tomography design, nearly all of the detectors are in operation at all times during the imaging procedure.
To date, two groups have espoused this approach and have built stationary ring-type, discrete detector systems for SPEC tomographic imaging. Rogers, W. L.; Clinthrone, N. H.; Stamos, J.; et al., "SPRINT: A Stationary Detector Single Photon Ring Tomography for Brain Imaging," 1 IEEE Trans. Med. Im. p. 63 (1982); and Hirose, Y.; Ikeda, Y.; Higashi, Y.; et al., "A Hybrid Emmission CT Headtome II," 29 IEEE Trans. Nucl. Sci. N.S. p. 520 (1982). For each of these designs, the major component subject to circular rotation is the collimator.
Rogers et al achieve fan beam geometery in SPEC tomography data sampling by using slit apertures in a ring collimator. Data sampling by each detector consists of viewing a small strip (ray) of the target volume through one of the slit apertures. As the ring collimator indexes in rotation, the rays sampled sequentially by a particular detector form a fan. The resolution performance is excellant (8 mm FWHM at the center) and varies only slightly in different regions of the imaging plane. However, the low intrinsic efficiency of slit apertures and the limited number of slits on the ring make the collimator very insensitive. In addition, the slit collimator design discriminates against activity at depth and favors activity at close range. These characteristics indicate that the great majority of the detected photons are from the peripheral area of the target volume.
The approach disclosed by Hirose et al. uses a varying pitch collimator. Data sampling during rotation of the collimator also yields a fan beam pattern for each detector. The collimator was designed for high efficiency by today's standard. However, resolution at the center of the imaging field (11.5 mm FWHM) is too low to quality this design as belonging to the high resolution design category. Moreover, the technical problems involved in fabrication of this collimator make it difficult to extend its resolution performance beyond 10 mm FWHM at the center of the imaging field.